Cloud providers offer the largest credits, but almost always require a card. The credit itself is free; the charges come later—when the credit runs out but resources keep running.
Set a budget limit on day one
Major cloud providers have spending alerts. Configure a threshold of a couple of dollars right after signup, before your first launch.
Write down the end date
Credit is valid for a limited time. Set a reminder for three days before it expires and decide in advance what you'll keep.
Delete resources instead of stopping them
A stopped instance often continues to charge for disk and IP address. If you no longer need a service, delete it completely.
Use a virtual card
A card with a low limit caps your losses if you forget to turn something off.